Is it legal to kill people fighting for Islamic State in Syria?

“To take them out using special services, I think, would be illegal under UK and international law.”

Jacob Rees-Mogg, 26 October 2017

“You can sometimes use lethal force. ... You can even do it on the street of your town in the UK, if it is strictly necessary to save life. That is lawful under English criminal law and under international law.”

Shami Chakrabarti, 26 October 2017

There’s a debate going on at the moment about what the government can and should do with British citizens in or returning from fighting in Syria for jihadist groups.

There is no single authoritative statement of what’s allowed in international law. It starts with written treaties agreed between countries, but also includes custom and practice. All of these can be interpreted differently by different countries and not all countries are signed up to all treaties.

The UK government has said that killing people in Syria can be lawful under international law. An RAF drone targeted and killed Reyaad Khan, a British national, in Syria in 2015.

The UK government’s view of international law was set out in a speech by the Attorney General earlier this year.

The UN Charter, the founding treaty of the United Nations, requires member states to “refrain in their international relations from the threat or use of force”. Article 51 makes an exception for “inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations”. That’s all in black and white.

“Like many other states”, the Attorney General said, the UK believes that right extends to “self-defence against an ‘imminent’ armed attack” not just to when a country has already been attacked.

In warfare, it's generally lawful to target enemy troops. But the UK government takes the view that international law may allow the use of force against individuals who are not members of a conventional foreign army.  It says force, which may be lethal force, may be used against these individuals if that's what's needed to defend the UK against an actual or imminent armed attack.

The UK's view is that it can use force in self-defence against individuals based in a foreign state if that state is unable or unwilling to prevent an actual or imminent armed attack on UK interests. The same would apply if the foreign state does not have effective control of the territory from which the attack might come.

The Attorney General says “a number of states” agree with the UK on that.

So those are the arguments you would need to accept to agree with to accept the UK government’s argument about what strikes can be legal in international law.

Finally, you would have to decide whether a particular strike is legal based on the facts of the case. Is it necessary? Is it proportionate? And what counts as an imminent threat?

The Security Service MI5 says that “hundreds of British extremists” have travelled to Syria. Its Director General said in a speech: “A growing proportion of our casework now has some link to Syria, mostly concerning individuals from the UK who have travelled to fight there or who aspire to do so.”